1. Field of the Invention
The present invention relates to a mobile communication base station antenna, in which simultaneous connection of plural users can be realized.
2. Related Art
Technique such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), or Code Division Multiple Access (CDMA) has been proposed to realize the simultaneous connection of the plural users in a base station to be used for mobile communication, and has been introduced into commercial systems.
However, as a result of sudden increase of mobile communication users in accordance with spread of the mobile communication for late years, there is a problem in that the number of frequencies (frequency source) becomes short due to call requests more than capacity of frequency channels assigned to the mobile communication system.
Therefore, the Space Division Multiple Access (SDMA), which realizes the communication with the plural users in one (single) frequency band, has been proposed so as to realize expansion of the channel capacity by increasing a utilization efficiency of the frequency. In the SDMA, the plural users are separated by difference in space, by turning a main beam orientation of a directivity of a base station antenna toward a desired user and turning a null orientation of the directivity of the base station antenna toward other users.
As a concrete technique for realizing the SDMA, there is a radio communication technique called as MIMO (Multiple Input Multiple Output), in which the channel capacity is increased by using plural antennas. In the MIMO, it is necessary to install plural antennas for dividing a transmission data into plural signals (streams) and simultaneously transmitting the divided signals.
Japanese Patent Laid-Open No. 2001-313525 (JP-A 2001-313525) proposes a mobile communication base station antenna for realizing the SDMA, in which plural array antennas are located linearly (on a straight line) or annularly (on a circumference of a circle).
In addition, K. Nishimori et al, “Channel Capacity Measurement of 8×2 MIMO Transmission by Antenna Configurations in an Actual Cellular Environment”, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, No. 11, November, 2006, pp. 3285-3291 proposes a mobile communication base station antenna for realizing the SDMA, in which four array antennas using V-H (vertical and horizontal) polarized wave and ±45 degree slant polarized wave are arranged in a horizontal direction.
In late years, in accordance with spread of mobile communication including portable telephone, the mobile communication base station antennas overflow all over the town. The mobile communication base station antenna is generally installed on a steel tower or a roof of a high building. Therefore, it is unfavorable that an installation occupied area (i.e. an area occupied for installation) of the mobile communication base station antenna in total is increased, since the increase in the installation occupied area raises the cost for installation or damages the landscape.
Further, it is also unfavorable that plural array antennas are arranged linearly along a longitudinal direction of an antenna, since such an arrangement may cause the problems in the installation or in the landscape.
Still further, as to the array antenna, since the number of antenna elements constituting an array antenna is determined based on an antenna directivity required in the mobile communication base station, it is impossible to reduce the number of the antenna elements even though there is the problems in the installation or in the landscape.
For the case of introducing the aforementioned MIMO technique, it is indispensable to arrange the plural array antennas so as to increase the channel capacity by improving the utilization efficiency of the frequency. However, as described above, it is unfavorable that the installation occupied area of the antenna in total is increased.
Accordingly, in recent years, it has been strongly desired to realize the mobile communication base station antenna which enables the SDMA without increasing the installation occupied area.